Individuals on probation, parole, or in alcohol treatment programs may be prohibited from consuming alcohol, and many federal, state, and local law enforcement agencies require testing to ensure participants in court ordered programs remain alcohol free. As alcohol is ingested orally, it is absorbed into the body's blood and distributed throughout the body via the circulatory system. Alcohol is eliminated from the body by two mechanisms: metabolism and excretion. Metabolism accounts for the removal of greater than 90% of the alcohol consumed, removing it from the body via oxidation of the ethyl alcohol molecule to carbon dioxide and water primarily in the liver. The remaining alcohol is excreted unchanged wherever water is removed from the body—breath, urine, insensible skin perspiration, and saliva. Although excretion accounts for less than 10% of the eliminated alcohol, it is significant because unaltered alcohol excretion permits an accurate measurement of alcohol concentration in the body by way of both breath analysis and insensible skin perspiration. Insensible skin perspiration is the vapor that escapes through the skin through sweating. The average person will emit approximately one liter of insensible skin perspiration each day. This insensible skin perspiration can be used to obtain a transdermal measurement estimating a blood alcohol concentration, referred to as Transdermal Alcohol Concentration (“TAC”).
Transdermal monitoring of alcohol levels is accomplished by taking measurements of alcohol contained in the insensible skin perspiration that is expelled transdermally through human skin. Throughout this description of the invention, insensible skin perspiration may be referred to as “vapor,” “air vapor,” “air vapor sample,” “air vapor volume,” “sample,” “sample volume,” “air sample” “transdermal vapor sample,” and “air sample volume,” interchangeably, with no difference in meaning intended. A monitoring device in the form of a bracelet that is worn on the ankle or arm of the subject captures the air vapor released from the skin under the bracelet and measures the alcohol contained therein, if any.
There are numerous advantages to transdermal alcohol monitoring, as opposed to breath-testing, including, but not limited to, the ability to take readings at any time without the knowledge of the subject, consistent and continuous testing (unlike breath alcohol testing where a subject breathing incorrectly into the testing device can cause inaccurate results), and the ability to convert such readings into electrical signals that can be transmitted to a central monitoring station.
The transdermal alcohol monitor described in co-pending application Ser. No. 12/013,931 better manages the build-up of moisture within a transdermal blood alcohol monitor to prevent damage to the various internal components, and to increase the service life of the transdermal alcohol monitor.
Regarding fixed-location breath-testing devices, a fuel cell is subjected to a breath sample having an alcohol vapor component. The physical measurement variable i(t), which is obtained by the electrochemical conversion, is supplied to an evaluation circuit which determines a measurement value proportional to the alcohol vapor concentration. The measurement value is determined by integrating the signal trace of the physical measurement value as a function of time (t). A sample curve of such a signal trace from a transdermal alcohol monitor is shown in FIG. 2. When the measuring cell is charged with alcohol vapor, the measurement signal first increases starting from a reference value, passes through a maximum value, and returns again to a minimum value in the vicinity of the reference value after the complete electrochemical conversion. The area enclosed between the function value of the measurement signal and the reference value is proportional to the concentration of the alcohol vapor in the breath sample. By looking at different sections of the area under the curve these methods determine different types of interferents in the breath sample that is being analyzed. An interferent may be defined as any substance whose presence interferes with the sample being measured and generates incorrect results. Thus, by utilizing the above techniques, true alcohol readings can be distinguished from the interferent readings.
However, these methods will not work with transdermal alcohol monitoring, as the interferent is different than in a breath test. The interferents in transdermal testing are of a significantly different concentration than a typical transdermal alcohol sample obtained from the skin. Interferents in transdermal testing will typically create a much different sample curve entirely and can thus be readily identified. However, occasionally, the concentration of the interferent is low enough to not create a completely different sample curve. In such cases, the interferent sampled in transdermal testing can generate a sample curve that looks very similar to a drinking event. Therefore a new method of detecting interferents in transdermal alcohol monitoring is needed.